This topic wasn’t addressed in other fog-related discussions, so here goes.
My windshield and my back window have lately been fogging up excessively, meaning more than other car’s windows in my parking lot. Were talking in the morning or evening here, not while I am driving, but after it sits. The back window also frequently fogs up while sitting in the lot at work, for about 5 hours. Again, no other cars around mine have fogged windows.
I have switched between the fresh and re-circulating air switches as mentioned elsewhere on this board, but that doesn’t seem to do any good. I know that my AC compressor is on the fritz. Could that be the problem?
have you had a heavy rain recently? If you have water in the car somewhere, fogging will happen. Check the carpets, etc.
You already stated the key. The AC compressor is not working. You need it to be working in order to dehumidify the air and remove moisture from the interior.
Your A/C will come on during the defrost mode, even in the winter, in order to dry out the air. If you have had moist weahter and your A/C is on the fritz, it is common to have too much moisture in the air in the car. When you park it, the car cools down and the moisture condenses on the windows. The cure for this if you have no A/C, is a long drive at highway speed with the heater on to get rid of the moisture.
Also keeping a window cracked open with the heat on will help get rid of the built up moisture inside the car.
I don’t know about you guys, but I don’t run my AC in the winter, except on those rare days of high humidity and rain. I realize many cars have the AC running automatically when the defrost in on, but many cars don’t.
Bottom line, only time I have had windows fogging up is when I left a window open and got moisture inside.
I agree with Bill on this. If you have the INSIDE of your windows fogging up, you have moisture inside somewhere. You may have a sunroof drain plugged, torn or missing door vapor barrier, leaking windshield, or leaking heater core. You may not feel the carpet wet due to the foam padding soaking up the moisture, but it’s in there somewhere. Check your trunk too. Moisture vapor can enter the cabin from the trunk. The car needs to be aired out to let the moisture evaporate.
“I don’t know about you guys, but I don’t run my AC in the winter” If you use the defog setting to direct air to the windscreen, you are using the A/C in most cars. It happens without you turning on the A/C.
You are getting and will get a variety of answers, and most all of them are right. Different conditions around the world mean different people will have different problems depending on where they live. Try all the solutions and see what works for you.
wrong. 03 Passat does NOT turn on AC unless you explicitly push the button. (I can use defrost AND push the AC button for fogging conditions)
Please re-read my reply. 
As Joseph mentioned ‘most’ cars, not ‘all’, but ‘most’ cars, do indeed turn on the a/c with the defrost even if you didn’t activate the a/c button. Your car doesn’t.
Thanks for the replies. It has been rainy lately, and this has been going on since about November, the time when rains start to kick in here in CA. Now that it is mentioned here, my rear window defroster does seem to be taking longer than usual to work.
I did the heater thing as recommended in this thread and that seems to have done the trick. Thanks
When you are home, if the car is always parked where there is shade, you will have a problem with that. The moisture never leaves the car. Hard sunshine helps, especially if you slightly crack a window on the sunny side.